The invention relates to a process and a supply system for the uninterrupted provision of liquid subcooled carbon dioxide at an essentially constant pressure greater than 40 bar.
In certain applications, large amounts of carbon dioxide at high pressure are required. An important aspect in this case is that the pressure is to be provided in as constant a manner as possible and the amount of carbon dioxide transported must be metered as accurately as possible.
Recently carbon dioxide uses are being established, for example, which require carbon dioxide at about 60 bar or above. For example, liquid carbon dioxide at 60 bar is required for foaming plastics, in supercritical extraction, in chilling, in plasma spraying using laminar nozzles or in charging small carbon dioxide vessels.
In the production of polystyrene foam (XPS) by the mechanical blowing process, the blowing agent carbon dioxide used as an alternative is forced into the foam extruder at up to about 350 bar using a diaphragm metering pump system. For the high pressure pumps, some manufacturers prescribe the use of room-temperature carbon dioxide which must be stored at a constant pressure and subcooled before entry into the metering pump.
To date, to provide liquid carbon dioxide at high pressure, a stationary high-pressure tank has been filled with cold carbon dioxide at low pressure (up to 20 bar). The carbon dioxide was then warmed, as a result of which the pressure in the high-pressure tank increased to the desired minimum pressure. During replenishment, the pressure had to be decreased back to the low pressure level. The pressure was decreased by releasing gaseous carbon dioxide from the high-pressure tank, which gave rise to costs and generally represented noise pollution for the environment. Furthermore, the supply with carbon dioxide was interrupted during the charging period. In order to avoid interruption of the carbon dioxide supply, two high-pressure tanks had to be mounted which were alternately charged and emptied. Not only the procurement costs of the two high-pressure vessels but also their maintenance costs due to the blow-off were considerable.
High-pressure storage in non-insulated heatable pressure vessels at 60 bar and 22° C. is not able to continuously ensure high-pressure conditions. Since tanker trucks for industrial scale carbon dioxide consumption always provide low-temperature low-pressure carbon dioxide (12 bar/−35° C.), the pressure in a high-pressure vessel collapses during replenishment. The supply pressure of the carbon dioxide must be elevated to the desired pressure level by an internal vessel heater having an output-dependent time delay.
Charging high-pressure carbon dioxide vessels using the customary tanker truck pumps also posed problems, so that the pressure in the vessels had to be released before charging to the maximum possible pump pressure.
Storage of low-temperature liquid carbon dioxide in a low-pressure tank and supplying a plant with liquid carbon dioxide at high pressure using a pump has the disadvantage that in the event of pump faults, supply of the plant with carbon dioxide is interrupted and thus gives rise to considerable costs.
It was also disadvantageous with known processes that carbon dioxide was always provided in a state close to its boiling point. Liquids close to their boiling point have a tendency to vapour formation, which makes metering more difficult and makes transport relatively energy-intensive owing to the compression losses which occur.
It is an object of the present invention, therefore, to specify an improved process and a supply system by which liquid carbon dioxide can be provided uninterruptedly and inexpensively at an essentially constant pressure greater than 40 bar.